1. Field of the Invention
The present invention generally relates to a projection optical system that projects a pattern on a reticle (mask) onto a substrate. More particularly, though not exclusively, the present invention relates to a catadioptric projection optical system including a lens and a reflecting mirror.
2. Description of the Related Art
Hitherto, a projection exposure apparatus that projects a circuit pattern on a reticle onto a wafer, serving as a substrate, by a projection optical system, and that transfers the circuit pattern onto the wafer has been used. In recent years, the required resolution of the projection optical system is becoming increasingly higher. To achieve the required high resolution of the projection optical system, it is necessary to either reduce the wavelength of exposure light or to increase a numerical aperture NA of the projection optical system.
At present, an ArF laser having a wavelength of 193 nm is used as exposure light. However, since only quartz and fluorite are available as glass materials that efficiently transmit light having a wavelength of 193 nm, it is very difficult to correct chromatic aberration. In addition, increasing the diameter of the glass material due to an increase in NA is a significant factor that increases the cost of the apparatus. In particular, in a projection optical system of an immersion exposure apparatus in which liquid is filled between the projection optical system and the wafer, NA is greater than 1. Therefore, an increase in the diameter of the glass material is a serious problem.
Accordingly, to overcome, for example, the problems regarding the difficulty in correcting chromatic aberration and an increase in the diameter of the glass material, various proposals involving the use of a catadioptric projection optical system, including a reflecting mirror, as a projection optical system have been made (refer to Japanese Patent Laid-Open No. 2001-228401 and the pamphlet of International Publication No. 2005/069055).
The catadioptric projection optical systems discussed in Japanese Patent Laid-Open No. 2001-228401 and the pamphlet of International Publication No. 2005/069055 have common features.
The catadioptric projection optical systems will hereunder be described with reference to FIG. 1. FIG. 1 is a schematic view of the catadioptric projection optical system discussed in Japanese Patent Laid-Open No. 2001-228401. In the projection optical system, a pattern of a first object 101 (reticle), disposed in an object plane, is focused onto a second object 102 (wafer), disposed in an image plane.
Here, a common feature is that the projection optical systems discussed in Japanese Patent Laid-Open No. 2001-228401 and the pamphlet of International Publication No. 2005/069055 each include a first image forming optical system and a second image forming optical system G1 (this common feature will hereunder be referred to as “feature A”). The first image forming optical system includes a reflecting mirror M1 and a concave mirror M2 and forms an intermediate image 180 which is an actual image. The second image forming optical system G1 forms an image of the intermediate image 180. Accordingly, the catadioptric projection optical systems discussed in Japanese Patent Laid-Open No. 2001-228401 and the pamphlet of International Publication No. 2005/069055 have the feature A. In addition, in each of these projection optical systems, an area of a reflecting surface of the reflecting mirror M1 where exposure light is reflected does not include an optical axis 103 of the projection optical system. Further, in each of these projection optical systems, a central portion of a pupil at the projection optical system is not shielded from light (or a pupil plane of the projection optical system is free of a void area).
In each of the catadioptric projection optical systems having the feature A, light from the first object can be reflected by an optical element (such as a lens), making up the image forming optical system G1, other than the reflecting mirror. In this case, light reflected by the optical element may be reflected again by the concave mirror M2 (whose reflecting surface opposes the first object 101) and converged, thereby producing a flare on the second object 102 near the optical axis 103. (The flare will hereunder be referred to as “flare (A).”) FIG. 2A shows a distribution of light that is projected onto the image plane when a slit illumination area in the object plane of the catadioptric projection optical system shown in FIG. 24 in Japanese Patent Laid-Open No. 2001-228401 is illuminated with light. In FIG. 2A, reference numeral 301A denotes an image in an illumination area, and reference numeral 302A denotes the flare (A). Referring to FIG. 2A, the flare (A) 302A is produced near the optical axis 103 so as to be separated from the image 301A in the illumination area. Similarly, FIG. 2B shows a distribution of light that is projected onto the image plane when a slit illumination area in the object plane of the catadioptric projection optical system shown in FIG. 30 in the pamphlet of International Publication No. 2005/069055 is illuminated with light. Referring to FIG. 2B, a flare (A) 302B is produced near the optical axis 103 so as to be separated from an image 301B in an illumination area. The flare (A) does not occur only in these projection optical systems. It also occurs in many other catadioptric projection optical systems having the feature (A).